Mechanical Motion Rectifier Based Single and Hybrid Input Marine Energy Harvester Analysis, Design and Basin Test Validation

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Virginia Tech

Point absorber style marine energy harvesters have been investigated based on their structure, energy harvesting efficiency, and reliability along with costs. However, due to the continuously varying ocean conditions and climates, the system usually suffers low power output and reliability from low input and high Peak to Average Ratio (PAR). Therefore, a Mechanical Motion Rectifier (MMR) based point absorber is introduced in this thesis to promote the harvesting efficiency and reduce the PAR by unifying the input rotation, and allow disengagement inside the gearbox during low power output phase. A 1:20 scale full system was then designed, prototyped, and tested based on the MMR. The bench test results show that the proposed MMR based point absorber could improve the energy conversion efficiency by 10 percent, which brings feasibility to the implementation. Traditional Wave Energy Converter(WEC) can only harvest ocean waves while ocean current is also one of the significant energy sources widely existing in ocean. In order to further increase the energy harvesting efficiency, one individual energy input source shows its limits. A vast majority of places around the world tends to co-exist both marine waves and current, and extracting energy from both sources could potentially increase the electric power output. Therefore, the Hybrid Wave and Current Energy Harvester (HWCEC) is introduced along with the hybrid gearbox. It is capable of harvesting energy from both ocean waves and current simultaneously so that the electric power output is significantly higher from a combined system. Tank test data shows 38-79 percent of electric power output promotion of an HWCEC compared to a regular WEC, and 70 percent reduced PAR in irregular wave condition. After that, system electric damping has been thoroughly investigated on both electrical side and mechanical side. The best power output corresponding electrical resistance is identical to the generator internal resistance while the best gear ratio of 3.5 is determined via both simulation and tank test. Furthermore, the system's PAR has been investigated by analyzing the trend of the peak occurrence. Tank test data shows the HWCEC's output power peak occurrence is at roughly 20 percent located at its PAR average. Therefore, the HWCEC system can promote energy harvesting efficiency to the combined system design, and improve its reliability from a significantly reduced peak to average ratio. It also gives HWCEC a large variety of deployable locations compared to a regular WEC under more marine environment. Furthermore, a new design of the Hybrid model, Hybrid LITE, is then developed, which not only features the HWCEC features, but also a lightweight, immersive and inflatable design for fast deployment and transportation. Since the system is built with an open water chassis, the overall system robustness is significantly improved since no water sealing is required on the powertrain compared to the HWCEC.

Marine Energy Harvester, Mechanical Motion Rectifier, Power Take-off, Wave Energy Converter, HWCEC